A popular type of window is the double-hung window having sashes that may be tilted relative to their frame to provide access to both sides of the sashes from only one side of the frame. However, the provision of such a window raises certain issues regarding structural integrity in view of the freedom between the sash and the frame necessary to allow pivoting of the former relative to the latter. There exists a need not only for a safe and reliable seal between the sash and the frame on such windows, but also for a window system that is cost effective to manufacture and convenient to operate.